Rechargeable electric storage devices have been used. The electric storage devices can be repeatedly used by charging and discharging. Therefore, the electric storage devices are eco-friendly in comparison to non-rechargeable batteries and thus currently used in various fields including electric vehicles.
A device using a plurality of electric storage devices may encounter a case where capacities of the electric storage devices become unequal due to fluctuations in initial capacity or deterioration rate of them. In charging, one or some of the electric storage device (s) sometimes reach a full-charge voltage earlier or later than the other electric storage devices. Therefore, not all of the electric storage devices can be charged fully. Further, in discharging, one or some of the electric storage devices sometimes reach an end-of-discharge voltage and stop being charged earlier or later than the other electric storage devices. Therefore, not all of the electric storage devices can be fully discharged and exhausted. Thus, if the capacities of the electric storage devices are unequal, those capacities cannot fully be utilized. Conventionally, a technology has been known of discharging the secondary batteries having unequal capacities by using a discharging circuit such as a resistor to equalize their capacities. According to the technology, remaining energy capacities of the secondary batteries are obtained from voltage information of the secondary batteries in a current-less state to discharge those secondary batteries based on their differences in capacity, thereby equalizing the capacities of the secondary batteries.
Recently, an olivine-type lithium-ion iron secondary battery (hereinafter referred to as an olivine-type iron battery) is attracting attention as a secondary battery for use in electric cars. The olivine-type iron battery is a kind of lithium-ion batteries and has a positive electrode including olivine-type iron phosphate and a negative electrode including, for example, a graphite-based material. Therefore, the olivine-type iron battery need not use a cobalt-based electrode as its electrode and has an advantage in that it is more inexpensive and safer than a secondary battery that uses a cobalt-based electrode.
The olivine-type iron battery has a plateau region extending from a point at which a state of charge (SOC) is at a certain level to a point at which the SOC is at another level. The plateau region is defined based on the olivine-type ion battery including a material of the negative electrode. The SOC indicates a remaining capacity of the secondary battery. If the negative electrode includes a graphite-based material, the plateau region may extend from a point at which the SOC is 10% to a point at which the SOC is 90%. In this context, the plateau region refers to a region where the voltage of a secondary battery stays roughly constant irrespective of changes in the SOC of the secondary battery. In the plateau region, it is difficult to estimate capacities of the electric storage devices from voltage information of those elements. Accordingly, to equalize the capacities of the electric storage devices by using the voltage information of those electric storage devices, a technology is desired to equalize the capacities of the electric storage devices by using a region other than the plateau region.
However, the olivine-type iron battery has a region (hereinafter referred to as variation region) in which a voltage sharply increases with respect to an increase in the remaining capacity. In the olivine-type iron battery having a negative electrode including a graphite-based material, regions in which the SOC is lower than 10% and higher than 90% may be the variation region. In the case of electric storage devices such as the secondary batteries having a variation region with such relatively high SOC, during charging for example, even if an attempt is made to equalize the electric storage devices by discharging the electric storage devices based on their voltage information acquired in the variation region, charging of the electric storage devices ends before their equalization ends. According to the conventional technologies, discharging of the electric storage devices would end upon the end of their charging. Thus, the electric storage devices could not be sufficiently discharged and the capacities of the electric storage devices are less likely to be equalized.